The present invention relates to a photoelectric conversion device and an image sensor applicable to facsimile, image scanner and digital copier, and to a method for forming the same.
Lately along the spread of facsimiles, a down-sized, light-weight and low price image sensor is being demanded. Image sensors used for facsimile, image scanner and digital copier, etc. are roughly divided into three types of non-contact type, contact type and full-contact type image sensors.
Currently, because the non-contact type image sensor using CCD projects an image of an original to the CCD through a reducing lens system, it is disadvantageous as compare to the other two systems in term of down-sizing and lightening, though it is advantageous in terms of price since it can be produced by a currently established LSI process using silicon wafers and since the CCD chip may be smaller.
On the other hand, although the contact and full-contact type image sensors are advantageous compare to the non-contact type in terms of down-sizing and lightening, their production cost is high due to difficulties in the fabrication process, packaging and assembly and the use of expensive parts such as a selfoc lens array and a thin plate glass has been being another large factor that hampers lowering of the price of those two type image sensors.
In concrete, mainly two types of contact type image sensors, i.e. a multi-chip type in which many MOSLSIs are packaged and a thin film type in which amorphous silicon thin films are used for its photo sensor part and are formed on an insulating substrate, are adopted and put into practical use for facsimiles.
A selfoc lens array, optical lenses that lead reflected light from an original to surface of a light receiving sensor, is used for them. The multi-chip type image sensor is created by LSI technology which is Today's most advanced technology, so that its yield is considerably high and it can be stably supplied. On the other hand, the production cost of the thin film type image sensor is high because a yield of its thin film semiconductor layer part is bad.
Although the full-contact type image sensor using thin film elements is the most advantageous especially in terms of down-sizing and lightening because it does not use the reduction lens system or the selfoc lens array, the bad yield of the thin film semiconductor layer part has been one of the large factors that hamper the lowering of the cost and it has been demanded to be able to provide the thin film semiconductor part stably with better yields. Moreover, as described before, the contact type image sensor also has the same problem when its photoelectric converting element part is composed of thin film elements.
As photoelectric converting elements of the image sensor, photoconductor type and photodiode type elements are known and being put into practical use. Generally the photoconductor type element has a characteristic that it is strong against noise since it can flow a large current, but its photo-responsibility is bad and it is disadvantageous to the demand for speeding up operations of facsimile. On the other hand, responsibility to light of the photo-diode type element is fast, though a current that flows in it smaller, and it is considered to become the main stream in the future.
FIGS. 2A and 2B show an upper plan view and a schematic section view of only a photoelectric converting element part of a prior art photo-diode type image sensor. In the figures, an insulating film (silicon oxide film) 21 that blocks alkali metal ions such as sodium is coated on a substrate 20. Provided on the insulating film 21 is a first electrode 22 on which an opening 26 for inputting light is created. Formed on the first electrode 22 is an amorphous silicon thin film semiconductor 23 which makes up a PIN type diode. Provided further on a photoelectric converting element area 27 of the thin film semiconductor 23 is an light transmitting electrode 24 and a metal electrode 25 is formed by being connected to the light transmitting electrode 24.
Normally conductivity of the thin film semiconductor becomes high when it is irradiated by light, so that a large part of light is blocked by the first electrode 22. To that end, the first electrode 22 is provided also at the part other than the photoelectric converting element part.
Because a thickness of this thin film semiconductor is less than 1 micrometer and it is fabricated by depositing films by means of vapor deposition and others, it has a high possibility that it does not function as a photoelectric converting element by causing a short or leak at a part 28 for example between the first and second electrodes 22, 25 due to contamination of dust and impurities.
Furthermore, when the photoelectric converting elements as shown in FIG. 2 are arranged one-dimensionally as shown in FIG. 3A to create an image sensor, a metal of the second electrode is left around an end portion 37 of the thin film semiconductor that exists between one photoelectric converting element 38 and an adjoining photoelectric converting element 39 and causes a short between those photoelectric converting elements.
That is, when such image sensor is created, acid treatment is sometimes carried out in a process after forming the semiconductor layer for example as a pre-treatment for connecting the metal electrode to an impurity area of TFT on the same substrate. Hydrofluoric acid is used as an acid for this treatment, so that the silicon oxide film 31 is etched around the end portion 37 of the thin film semiconductor layer and a concave portion is formed. Then a metallic layer is formed on the whole surface and is patterned to create the metal electrode 35, and the metal 36 is left at the concave portion, causing a short or leak between the photoelectric converting elements 38 and 39.
Many electric shorts or leaks are caused inside or between photoelectric converting elements, thereby reducing the production yields of photoelectric conversion devices and image sensors.
Furthermore, such failures as reduction or coloring of the transparent conducting films 24 and 34 are caused in the manufacturing process due to a fluid for releasing resist or a fluid for etching the metallic thin film wirings, decreasing the production yields further.